Field effect transistors are typically manufactured on a semiconductor substrate that is doped to contain mobile electric charge carriers. When incorporated into a semiconductor substrate lattice as a result of an activation process, dopant atoms can be either electron donors or acceptors. An activated donor atom donates weakly bound valence electrons to the material, creating excess negative charge carriers. These weakly bound electrons can move about in the semiconductor substrate lattice relatively freely, facilitating conduction in the presence of an electric field applied by a gate terminal. Similarly, an activated acceptor produces a mobile positive charge carrier known as a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. Common n-type donor atoms used in conjunction with silicon, semiconductor substrates include arsenic, phosphorus, and antimony.
The dopant implant or in-situ dopant growth parameters used for semiconductor substrate doping of the doped layers beneath the gate are key to optimum performance of the FET device with respect to important parameters, such as threshold voltage or channel mobility. However, limitations in implant tools, required thermal processing conditions, and variations in materials or process can easily result in unwanted diffusion of dopant materials away from the initial implanted position, decreasing performance or even preventing reliable transistor operation. This is particularly true when co-dopant implant processes are used, since different dopant types have different solid diffusion constants and respond differently to process conditions.
Cost effective electronic manufacturing requires transistor structures and manufacturing processes that are reliable at nanometer scales, and that do not require expensive or unavailable tools or process control conditions. While it is difficult to balance the many variables that control transistor electrical performance, finding suitable transistor dopant structures and manufacturing technique that result in acceptable electrical characteristics such as charge carrier mobility and threshold voltage levels are a key aspect of such commercially useful transistors.